A basic step in the catalytic mechanism of sulfhydryl protease action involves nucleophilic attack of a highly reactive active site sulfhydryl group on polypeptide substrates resulting in the formation of a thioacyl-enzyme intermediate. The project will test the hypothesis that the high reactivity of the sulfhydryl is due to formation of a thiolate-imidazolium ion pair between an active site imidazole function and the sulfhydryl group. A common mechanism for sulfhydryl protease action is proposed, based on this ion pair, which will be the subject of experimental testing.